Valve to piston clearance
Discussion
Hello all,
Does anyone know the valve to piston clearance for a stock 1275 A series engine?
I am trying to rebuild and modify my 1970 AH Sprite (OK not a Mini but I used to have two of those 20 years ago and still regret having to sell them
; anyone got a good one out there?) engine and I noticed a few people here (e.g. Cooperman) mentioned squish. My friend (who races motorbikes and is currently rebuilding his R1) will help me in the rebuild but he's going on about the squish all the time. So it would be useful to know the valve to piston clearance for the 1275 A-series.
Thank you
Does anyone know the valve to piston clearance for a stock 1275 A series engine?
I am trying to rebuild and modify my 1970 AH Sprite (OK not a Mini but I used to have two of those 20 years ago and still regret having to sell them
; anyone got a good one out there?) engine and I noticed a few people here (e.g. Cooperman) mentioned squish. My friend (who races motorbikes and is currently rebuilding his R1) will help me in the rebuild but he's going on about the squish all the time. So it would be useful to know the valve to piston clearance for the 1275 A-series. Thank you
Valve to piston clearance is not a problem, unless the timing chain snaps(!!).
To check this is really quite easy, if it's something you feel you want to quantify for your own peace of mind.
When you build the engine, machine the block to bring the pistons virtually flush with the block deck at TDC. You do this by carrying out a 'trial' or 'dummy' build before final assembly of the bottom end and short motor.
Knowing which cam you are using will tell you the amount of valve lift at max opening. So, you set the head up on its side and use a dial guage on the valve face. The valve springs are not to be fitted at this stage. Open the valve until the DTI gives a reading equal to the max opening and see how the valve face compares to the underside level of the head.
Bear-in-mind the compressed gasket thickness (measure this) and add it to the head level when comparing valve position to head face (actually gasket face). If the valve still protrudes beyond the gasket underside datum, then you need to look at the piston to valve opening timing (more complicated), but it's unlikely to come to this unless you are using 1.5:1 or 1.7:1 rockers. The normal cam lift to valve opening ratio is 1.3:1.
I'm sorry if this sounds a bit complicated. Email me if you need any clarification of this. I hope it helps,
Peter
To check this is really quite easy, if it's something you feel you want to quantify for your own peace of mind.
When you build the engine, machine the block to bring the pistons virtually flush with the block deck at TDC. You do this by carrying out a 'trial' or 'dummy' build before final assembly of the bottom end and short motor.
Knowing which cam you are using will tell you the amount of valve lift at max opening. So, you set the head up on its side and use a dial guage on the valve face. The valve springs are not to be fitted at this stage. Open the valve until the DTI gives a reading equal to the max opening and see how the valve face compares to the underside level of the head.
Bear-in-mind the compressed gasket thickness (measure this) and add it to the head level when comparing valve position to head face (actually gasket face). If the valve still protrudes beyond the gasket underside datum, then you need to look at the piston to valve opening timing (more complicated), but it's unlikely to come to this unless you are using 1.5:1 or 1.7:1 rockers. The normal cam lift to valve opening ratio is 1.3:1.
I'm sorry if this sounds a bit complicated. Email me if you need any clarification of this. I hope it helps,
Peter
Squish is really important with 2 strokes and perhaps that has gone into 4 stroke bikes as well... From what I gather, squish is important for getting the maximum out of a race engine or a very high revving one, if you were to plot a graph of pressure against time, as the piston rises, there is a sudden spike in the pressure which also aids energy transfer, less time for detonation, more power produced for a given fuel, it's like having a higher compression ratio....Anyway, I think it has relative importance due to the shielding effect of the top ring. It's more complicated than that though, you have burn speed, combustion chamber shape, etc, etc to think about. It's all interrelated. Anyway, the turbo guys go on about it (when scooping out their chambers) and it definately holds true. Factory engines were hardly near the blueprint and so fell off in this manner.
If you read a 2 stroke tuning book, it will all make sense. Stu
If you read a 2 stroke tuning book, it will all make sense. Stu
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